We have raised a serum in rabbits to human common acute lymphocytic leukemia cells (cALL). Following absorption with B cells, the serum is cytotoxic for cALL cells and not for normal B or T lymphocytes. The serum react with a cALL antigen of 98,000 daltons. We also have produced a hybridoma cell line which secretes a cytotoxic monoclonal antibody of the same specificity. We have also produced a cytotoxic monoclonal antibody against an acute myclocytic leukemia (AML) antigen. The antibody does not react with normal peripheral blood white cells, but it does react with mycloid bone marrow stem cells. We have developed a panel of heteroantisera that can subclassify the type of leukemia into diagnostically meaningful subtypes. The antisera react with the following antigens: la-like, T cell, blast cell, cALL and AML. We have raised heteroantisera to subpopulation of peripheral blood T lymphocytes and that reacts with two different cell membrane antigens.